1. Field of the Invention
The present invention concerns the aerodynamic profiles used to generate lift on rotorcraft and is more particularly concerned with a family of profiles for helicopter rotor blades.
2. Description of the Related Art
An aerodynamic profile is generally defined by a table of dimensions. This sets out:
a fixed maximal thickness,
the position of this maximal thickness is fixed,
a fixed maximal relative camber, and
the position of this maximum camber is fixed.
From a given profile any process can be used to generate profiles having different relative thickness to form a family of profiles. The process employed may or may not preserve the position of the maximal thickness, the value of the camber and its position.
In the case of helicopter rotor blades the combination of the rotor rotation speed and the speed of the helicopter generates at the upwind blade (blade azimuth angle "psgr" in the range 0xc2x0 to 180xc2x0) relative Mach numbers varying from approximately 0.3 at the blade root to 0.85 at the blade tip and at the downwind blade (blade azimuth angle "psgr" in the range 180xc2x0 to 360xc2x0) much lower Mach numbers from 0.4 at the blade tip to 0 or even negative values (profiles leading by the trailing edge) in the inversion circle near the rotor hub.
Because of this the kinetic pressure varies along the blade and in accordance with the blade""s azimuth position. To balance the aircraft it is necessary for the lift Cz and consequently the angle of incidence to be low at the upwind blade and high at the downwind blade. During one rotation of the blade the profiles constituting it alternately encounter high relative speeds and low angles of incidence and then moderate relative speeds and high angles of incidence. The speed (or Mach number) and the angle of incidence encountered by the profile depend on their spanwise position along the blade.
To define blades having high performance, that is to say minimising the power needed to rotate the rotor and/or enabling the aircraft to fly with an extended flight envelope in terms of lift and speed, it is necessary to use profiles offering high performance in all operating conditions. To minimise torsion forces on the blade and control forces on the pitch control rods, these profiles must have very low pitch moment coefficients throughout their range of operation. Finally for structural reasons, it is beneficial to construct blades whose thickness varies spanwise, being thicker at the root end and thinner at the tip.
Designing a helicopter blade offering high performance therefore requires a family of profiles, each profile having geometrical and aerodynamic characteristics that are well suited to the operating conditions encountered according to its spanwise position along the blade. The family of profiles must also be homogenous, that is to say all the profiles must have similar levels of performance as otherwise the performance of the blade will be limited by the performance of the worst profiles.
Various profiles or families of profiles are described, in particular in U.S. Pat. No. 3,728,045 (Balch), U.S. Pat. No. 4,142,837 (De Simone), U.S. Pat. No. 4,314,795 (Dadone), U.S. Pat. No. 4,569,633 (Flemming) and U.S. Pat. No. 4,744,728 (Ledciner) and in patent EP-0 517 467 (Nakadate). In the patents describing families of profiles the latter are generated from a base profile using two different processes:
the first process consists in defining the profiles using a unique camber law or skeleton and a law of thickness relative to the maximal relative thickness. This technique is described in xe2x80x9cTheory of wing sectionsxe2x80x9d by H. Abbot and E. Von Doenhoff published by McGraw-Hill in 1949. Profiles of different thickness are obtained by applying a multiplier coefficient to said thickness law which is therefore the same for all the profiles;
a second process, starting from a base profile whose extrados and intrados Y-axis coordinates are defined by a table of values, consists in defining the other profiles by applying a multiplier coefficient to these coordinates, the multiplier coefficient possibly being different for the extrados and for the intrados.
The Balch and De Simone patents define profiles having a relative thickness around 10% which cannot be used to define a blade with an evolving profile having performance adapted to suit its spanwise position.
The Dadone patent describes a family of profiles obtained by the second process.
The Flemming patent describes a family of profiles that can be generated by either of the two processes.
The Ledciner and Nakadate patents describe families of profiles generated by the second process in both cases.
However, the families of profiles like those described above do not yield profiles having geometrical characteristics adapted in accordance with the relative thickness, i.e. profiles all offering high performance, and consequently do not yield high performance blades. Accordingly, in the families described previously, the position of the maximum camber and the value of the maximal camber do not vary with the relative thickness or vary in a non-optimal manner. Likewise, the relative thickness between 20% of the chord and the position of the maximal thickness does not change with the relative thickness or changes non-optimally. Thus these families of profiles do not yield high performance blades even if the base profile of the family offers good performance because the performance of the blade is limited by the non-optimal performance of the derived profiles.
The family of profiles described in patents FR-2 463 054 and FR-2 485 470 have certain features, in particular the change in thickness between 20% of the chord and the position of the maximal thickness, which confer high performance on this family at low lift and high Mach numbers. On the other hand the cambers do not have features enabling the Cz levels to be increased significantly without reducing trans-sonic performance.
An aim of the present invention is to avoid these problems.
To this end, the blade profile in accordance with the invention for rotorcraft rotors comprising, between a leading edge and a trailing edge, an extrados and an intrados the camber of which is defined by the geometrical locus of points equidistant from them is noteworthy in that the ratio of the maximal camber to the maximal thickness varies in a linear fashion with the relative thickness of the profile and is in the range 0.13 to 0.19 for a relative thickness of 7% of the chord and is in the range 0.18 to 0.24 for a relative thickness of 15% of the chord.
The law of evolution of the ratio of the maximal camber to the maximal thickness as a function of the relative thickness is advantageously represented by the equation:                     (                  c          /          C                )            max                      (                  e          /          C                )            max        =            a      3        +                            b          3                ⁡                  (                      e            /            C                    )                    max      
the values of the coefficients a3 and b3 being as follows:
a3=0.1177,
b3=0.6114.
Moreover, the position of the maximal camber evolves in a linear fashion with the relative thickness of the profile and is in the range 14% to 16% of the chord for a relative thickness of 7% and in the range 27% to 29% of the chord for a relative thickness of 15%.
The law of evolution of the position of the maximal camber is preferably represented by the equation:             Xc      max        C    =            a      2        +                            b          2                ⁡                  (                      e            /            C                    )                    max      
the values of the coefficients a2 and b2 being as follows:
a2=0.0321,
b2=1.6499.
As explained in more detail below, these particular features of the camber and its position as a function of the relative thickness yields good performance in terms of maximal lift well matched to their position on the blade for all the profiles of the family in accordance with the invention.
Moreover, the ratio between the thicknesses at 20% of the chord and the maximal thickness varies in a linear fashion with the relative thickness and is in the range 0.957 to 0.966 for a relative thickness of 7% and in the range 0.938 to 0.947 for a relative thickness of 15%.
The law of evolution of this ratio advantageously is represented by the equation:                               (                      e            /            C                    )                ⁢                  X          /          C                    =              20        ⁢        %                            (                  e          /          C                )            max        =            a      1        +                            b          1                ⁡                  (                      e            /            C                    )                    max      
the values of the coefficients a1 and b1 being as follows:
a1=0.9779,
b1=xe2x88x920.2305.
Moreover, the blade profile in accordance with the invention is characterised by a maximal relative thickness position in the range 31% to 35% of the chord.